Developments in low pressure fluid bed gas phase polymerization and copolyermization of ethylene began to appear in the patent literature over a decade ago. Prior to these developments low density polyethylene was produced at high pressure, at least 15000 psi, homopolymerization of ethylene in the gas phase in stirred and elongated tubular reactors in the absence of solvents using free radical initiators. On a world wide basis, the amount of low density polyethylene produced in this fashion, annually, amounted to more than thirteen billion pounds.
Then it was discovered that the low density polyethylene could be produced commercially at pressures of less than 1000 psi in a gas phase reaction in the absence of solvents by employing selected chromium and titanium (and, optionally fluorine) containing catalysts under specific operating conditions in a fluid bed process. The products of those original processes exhibited broad molecular weight distribution and thus were limited in their application to wire and cable insulation and pipe production. Because of poor optical and mechanical properties, the products were not broadly used in film applications.
To be commercially useful in the gas phase fluid bed process, the catalyst therefor must exhibit high activity, with concomittant high product productivity, because gas phase process systems do not usually include catalyst residue removal procedures. Accordingly, catalyst residue in the polymer product must be so small that it can be left in the polymer without causing any undue problems in the fabrication and/or to the ultimate consumer, as reported in U.S. Pat. No. 4,302,566. To this end, the patent literature is replete with developments of new catalysts, of high activity with correlative high productivity values.
Certain low pressure fluid bed gas phase processes employing high activity catalysts produce high levels of gel in the polyethylene homopolyer and copolymer product. These gels are very high molecular weight species. The presence of gels in the polymer product causes poor film appearance and, in severe cases, causes unsatisfactory performance in end-use applications. Such problems have been experienced in both blown-films and blow-molded bottles.
An independently, although concurrent, observable result is fouling of the distributor plate in the fluid bed reactor. Fouling of the distributor plate in the fluid bed reactor reduces the ability to operate the reactor continuously.
Accordingly, it is an object of the invention to substantially reduce, preferably to substantially eliminate the gels found in the polymer products produced in the low pressure fluid bed gas phase polymerizations of ethylene.
Accordingly, it is another object of the invention to substantially reduce, preferably to substantially eliminate fouling of the distributor plate in the fluid bed reactor, since fouling of the distributor plate in the fluid bed reactor reduces the ability to operate the reactor continuously.